The robotics field and the automated processes field employ optical position sensors to obtain position information about workpieces, equipment, and the like. One application of optical position sensors is in the field of robotic welding. In robotic welding equipment, one or more optical position sensors may be used to determine the location and orientation of the workpiece in robot coordinate space. Robot coordinate space is a frame of reference in which a robotic controller defines location and orientation. For example, a robot controller may use a Cartesian coordinate system as robot coordinate space, wherein each point in space is defined by three variables (X, Y, Z).
Optical sensors obtain location and orientation information using one or more measurements of displacement between the sensor and the object being measured. Displacement measurements not only provide relative location, but also orientation and shape information when a sufficient number of data points on the object have been measured. For example, it is known that three displacement measurements of a planar surface define the precise location and orientation of that surface. In other words, if the distance from three known points to three points of a planar surface are measured, the orientation of the planar surface may be determined.
A common type of sensor is a reflective light sensor, such as the reflective light sensor 10 shown in FIG. 1. The reflective light sensor comprises a light source 11 and light detector 12 located on a sensor housing 14. The light source 11 generates a light beam 16 that is intended to strike the object to be measured, such as a surface 18, at an angle slightly less than orthogonal or 90.degree.. The light 16 reflects off of the surface 18 and strikes the light detector 12. The distance or displacement between the housing 14 and the surface 18 can be determined by the position on the detector 12 upon which the reflected light strikes the detector 12.
The accuracy of the above described measurement depends in part on the assumption that the sensor housing 14 is aligned in a predetermined orientation with respect to the surface. If the orientation of the sensor housing 14 misaligned with respect to the surface, the measured distance to surface 18 will be inaccurate. This point may be illustrated by considering a rotation of the surface 18 in FIG. 1 about a horizontal axis that includes the point where the light beam 16 intersects the surface 18. With only slight rotation, it is apparent that the reflected light beam will strike the light detector 12 in a different spot while the actual distance to the intersection point does not change. Because the reflected light beam strikes a different spot on the detector 12, the reflective light sensor 10 will calculate a different and inaccurate distance. Accordingly, proper alignment of the sensor housing 14 is an important factor in achieving accurate measurements.
A more significant drawback of the reflective light sensor 10 shown in FIG. 1 is that the sensor 10 is only capable of taking a distance measurement in a single orthogonal direction with respect to the housing 14. Specifically, the reflective light sensor 10 can only take measurements in the horizontal or "x" direction. This drawback, combined with the requirement of proper alignment, may prevent or restrict use of the sensor in many applications.
For example, consider an automated process involving the measurement of a plurality of surfaces that are oriented in a plurality of directions. In particular, consider a robotic welding process in which a first surface is to be welded onto a second surface to form a fillet weld. In such a situation, a reflective light sensor may be used to determine the location of the weld spot in a process known as corner location. In corner location, the reflective light sensor calculates the distance from a reference point to each surface and then applies algebraic relationships to determine the location of the intersection of the surfaces. To measure the distance to each of the two surfaces, the sensor 10 of FIG. 1 must be aligned to a predefined orientation with respect to the first surface, take the measurement, and then be realigned to a predefined orientation with respect to the second surface and take the measurement. The realignment is both time consuming and may be mechanically difficult to achieve due to space constraints. Moreover, realignment of the reflective light sensor introduces a source of error in the corner location calculations because of tolerances inherent to each alignment operation. It would therefore be useful to have a reflective light sensor in such a situation that could perform orthogonal measurements without realignment.
U.S. Pat. No. 5,448,359 to Schick et al. shows an optical distance sensor that includes means for scanning a measurement light beam with respect to the sensor housing. In particular, the Schick et al. device includes a polygonal mirror placed within the path of the measurement beam. The polygonal mirror rotates to effectuate beam scanning. The use of the rotating mirror, however, limits the angular range of beam deflection. The sensor must still be arranged more or less in one particular predefined orientation with respect to the object to be measured.
A need, therefore, exists for a reflective light sensor that can more easily measure a plurality of differently oriented surfaces.